The subject matter disclosed herein relates generally to nuclear medicine (NM) imaging systems, and more particularly to systems and methods for calibrating the detectors of NM imaging systems.
NM imaging systems, for example Single Photon Emission Computed Tomography (SPECT) and Positron Emission Tomography (PET) imaging systems, use one or more image detectors to acquire image data, such as gamma ray or photon image data. The image detectors may be gamma cameras that acquire two-dimensional views of three-dimensional distributions of emitted radionuclides (from an injected radioisotope) from a patient being imaged.
In single photon imaging systems, such a planar or SPECT imaging systems, collimators may be placed in front of a scintillation crystal or solid state detector to focus the field of view (FOV) of the detectors. The collimators allow gamma rays aligned with the holes of the collimators to pass through to the detector. These detectors need to be calibrated, including during manufacture and periodically after installation to ensure proper imaging operation. For example, the detectors are calibrated to provide a uniform energy and sensitivity response across the detector units or output channels.
Calibration of these collimated detectors is performed using a calibration emission source that exposes the detectors, and more particularly the entire detector or array of detectors, to radioactive emissions. Accordingly, in pixelated detectors, each pixel is exposed to a statistically relevant number of photons. In some collimated detector systems, calibration with multiple isotopes (having different energy peaks) is not practical after the system is assembled. Accordingly, in collimated detectors, the calibration is performed on the detectors having the collimators removed and using a jig or guide. This enables a two point energy calibration (gain and offset), wherein the calibration uses at least two isotope sources with two different peaks for calibration. A point source, however, cannot be used on the assembled system as the collimators at least partially obscure some parts of the detector. Thus, because the collimators need to be removed for calibration, detector calibration can easily be performed at the factory, but not after installation, for example, at a medical site, such as if parts of the detector are replaced.
Moreover, the presence of the collimators, such as in the assembled imaging systems, allows the use of flood sources only to expose or illuminate the entire detector, namely all the pixels, for calibration. Flood sources are difficult to handle and use. Additionally, the flood source can cause problems and have safety issues due to the significant radiation emitted by these flood sources. Flood sources are also more expensive than point sources and generally are available for only a limited number of isotopes. Shielding a large flood source is heavy and cumbersome. Generally, a hospital or a field serviceman may have one flood source for performing quality control and field calibration procedures.